1. Field of the Invention
This invention relates in general to video display devices which provide a very large picture screen and in particular to an improved driving system for such device.
2. Description of the Prior Art
A video display device has been proposed which uses a large number of luminescent display cells having so-called phosphor trio elements formed of, for example, red, green and blue phosphor layers which are arranged so as to provide a very large picture screen.
As illustrated in FIGS. 18A and B, such luminescent display cell is formed such that the red, green and blue three phosphor layers 2 (2R, 2G and 2B) are surrounded by a carbon layer 3 and are deposited on the inner surface of a front panel 1A of a glass housing 1. Three wire cathodes K (K.sub.R, K.sub.G and K.sub.B), first grids (control electrodes) G.sub.1 (G.sub.1R, G.sub.1G and G.sub.1B) and a common second grid which is the accelerating electrode G.sub.2 are provided in opposing relationship to the color phosphors 2R, 2G and 2B.
The color phosphor layers 2R, 2G and 2B are respectively surrounded by a separator 4 and the electron beams from the respective wire cathodes K are radiated toward or directed individually toward the corresponding phosphor layers 2 In this arrangement, an anode terminal 5 through which an anode voltage is supplied to the phosphor layers 2 is lead out through the separator 4 and between the front panel 1A and a side plate 1C of the glass housing 1 Terminals 6 are provided for the cathodes K, the first grid G.sub.1 and the second grid G.sub.2 and are lead out between a rear panel 1B and the side plate 1C. In the luminescent display cell, the anode voltage is supplied through the anode terminal 5 to the phosphor layer 2 with the voltages at the anode side and the second grid G.sub.2 being fixed and the luminescent display cell is selectively turned on and off by the voltage applied to the first grid G.sub.1.
In the above described apparatus, when a picture screen has 144 rows and 192 columns arranged in a matrix, it is required to have a total of 27,648 display cells (144 rows .times.192 columns). Also, the number of color elements will be three times that of the number of luminescent cells which will be about 83,000.
In such display devices, different display signals are supplied to each of the 83,000 color elements at the frame rate so as to produce the display in response to the respective display signals. Since the display of each color element comprises the on and off display as described above, the brightness modulation is accomplished with pulse width modulation PWM which controls the on period in response to the display signal supplied.
For this case, however, since the drive circuit including the above-mentioned PWM circuit and the like is relatively expensive because if a drive circuit is provided for each of the 83,000 color elements or luminescent display picture elements, the display device will be bulky and complicated and the price of the device is high. Also, the luminescent display cells generate a large amount of heat during the display operation and if the luminescent display is continuously produced, the luminescent display cells will become very hot Thus, this can cause breakdown of the device due to the high temperatures.